add note about logging keyword

Signed-off-by: Alex Boten <aboten@lightstep.com>

specification/configuration/sdk-environment-variables.md

@@ -236,7 +236,7 @@ Known values for `OTEL_TRACES_EXPORTER` are:
 
 - `"otlp"`: [OTLP](../protocol/otlp.md)
 - `"zipkin"`: [Zipkin](https://zipkin.io/zipkin-api/) (Defaults to [protobuf](https://github.com/openzipkin/zipkin-api/blob/master/zipkin.proto) format)
-- `"console`: [Standard Output](../traces/sdk_exporters/stdout.md)
+- `"console`: [Standard Output](../trace/sdk_exporters/stdout.md)
 - `"none"`: No automatically configured exporter for traces.
 
 Known values for `OTEL_METRICS_EXPORTER` are:
@@ -252,6 +252,9 @@ Known values for `OTEL_LOGS_EXPORTER` are:
 - `"console`: [Standard Output](../logs/sdk_exporters/stdout.md)
 - `"none"`: No automatically configured exporter for logs.
 
+Note that `"logging"` is a reserved keyword and shouldn't be assigned for future use. Implementations that use `"logging"` as a value to configure the
+standard output exporter MAY continue to do so for backwards compatibility.
+
 ## Metrics SDK Configuration
 
 **Status**: [Mixed](../document-status.md)